Predicting the Tides

This might seem boring to some, but I seem to making some progress in recognizing tide patterns. This is probably very elementary to those who've spent a lot of time near the Bay, but it's really been a mystery to me. Even before moving to the Bay I found the subject interesting and read everything I could find about it. Still, it wasn't until the last few weeks that I really learned some of the secrets. First of all, I had to stop looking at the on-line tide calculators. It's not that they aren't accurate, it's just the the tides where I live are nothing like those at the closest predicting station which is Sandy Point. The reason is that Sandy Point is on the main channel of the Chesapeake, and our house is about a mile away inside the mouth of the Magothy. I started imaging the tide as a giant flat wave pouring in from the Atlantic. It pours up the Bay and even past our river on up to Baltimore before it reaches our dock. Depending on the size and speed of the wave, we can be as much as a couple of hours behind the channel, even though it's clearly visable. The moon and, to some extent the planets, influence the wave. We get the biggest, fastest wave when the moon is full. Considering only these factors, we get a usual fluctuation of about 18 inches. There is an ebb and flow twice a day. At least at this time of year, the highest tide usually comes in the morning after sunrise.

That's my theory, except there's a big problem. Since we moved here, I've seen fluctuations up to 4 feet! How can that be? The culprit is the wind. A steady wind blowing north-northwest or up the Bay can either push the "tidal wave" along and make it bigger, or create its own wave independent of the lunar tide. A southerly wind will have the opposite effect. Normal breezes of 5 to 15 mph (I still haven't made the mental conversion to knots.) don't have much effect. But a steady blow in the 20s will make a big difference in the water level. According to my very non-scientific observations, sustained winds in the 30s can vary the levels by more than a foot. The biggest changes occur when a low pressure system moves directly Southwest of us. Since these system rotate counter-clockwise, (picture the arrows on a weather map) they blow the water directly up the Bay from the ocean. Once the wave moves up the Bay, the water actually stacks up creating much higher levels in the upper reaches. The longer the wind blows, the more it stacks up.

That's how I predicted Monday morning's high tide, the second highest since we moved here. I already knew we had a full moon and that the water would be high. I also noticed that a big low pressure system had stalled out just inside the coast of North Carolina. Even though the winds weren't that strong here, I knew they must blowing hard to our south. Sure enough, we awoke Monday to see the Chesapeake Bay nearly in our driveway. It probably seems silly, but I was very happy to see that my prediction had come true. (I made it out loud to Dianne, so she can verify my story!) But then, driving to work Monday, I nearly spilled my coffee in my lap when I realized something else. Hurricanes are low pressure systems, and when they come in from the Atlantic, they generally move right up the coastline. We might actually be better off to get a direct hit rather than a glancing blow to the south. A storm surge of 2 feet above Monday's high water-level would easily flood our ground floor. Even a tropical storm could do that. I wonder if we can buy flood insurance? -S